C Compare String: A Detailed Guide to strcmp()

In the realm of C programming, strings are fundamental data types used to represent text. Comparing strings is a common operation, and C provides a built-in function called strcmp() to handle this task efficiently. This article will delve into the intricacies of the strcmp() function, exploring its syntax, working mechanism, and practical applications with illustrative examples, making it an essential guide for anyone working with strings in C.

Understanding the Syntax of strcmp()

The strcmp() function is a part of the standard C library and is declared within the <string.h> header file. To utilize it in your C programs, you must include this header file.

The syntax for strcmp() is straightforward:

strcmp(s1, s2);

Parameters:

• s1: A pointer to the first string (char array).
• s2: A pointer to the second string (char array) to be compared with the first string.

Return Value:

The strcmp() function compares the two strings s1 and s2 lexicographically and returns an integer value based on the comparison:

• 0 (Zero): Returned when both strings are identical. This means they have the same sequence of characters.
• Greater than 0 (Positive): Returned if the first string s1 is lexicographically greater than the second string s2. This indicates that the first differing character in s1 has a higher ASCII value than the character at the same position in s2, or if s1 is a longer string and s2 is a prefix of s1.
• Less than 0 (Negative): Returned if the first string s1 is lexicographically smaller than the second string s2. This signifies that the first differing character in s1 has a lower ASCII value than the character at the same position in s2, or if s2 is a longer string and s1 is a prefix of s2.

How strcmp() Function Works: Lexicographical Comparison

The strcmp() function operates by comparing two strings character by character, based on their ASCII values. This type of comparison is known as lexicographical comparison, similar to how words are ordered in a dictionary.

Here’s a step-by-step breakdown of how strcmp() works:

1. Character-by-Character Comparison: The function starts by comparing the first character of both strings.
2. ASCII Value Check: It compares the ASCII values of these characters.
3. Continuing Comparison:
   • If the characters are the same, strcmp() proceeds to compare the next characters in both strings.
   • This process continues until either a mismatch is found or the end of one or both strings (null terminator ) is reached.
4. Null Terminator Encountered: If strcmp() reaches the null terminator in both strings simultaneously, it means the strings are identical, and the function returns 0.

5. Mismatch Found: If strcmp() encounters characters that are different:
   • It determines which character has a higher ASCII value.
   • If the character in s1 has a higher ASCII value, strcmp() returns a positive value (greater than 0).

-   If the character in `s1` has a lower ASCII value, `strcmp()` returns a negative value (less than 0).

Practical Examples of strcmp() in C

Let’s illustrate the behavior of strcmp() with several practical code examples.

Example 1: Comparing Identical Strings

This example demonstrates the case where strcmp() compares two identical strings.

#include <stdio.h>
#include <string.h>

int main() {
    char s1[] = "Hello";
    char s2[] = "Hello";

    int result = strcmp(s1, s2);

    if (result == 0) {
        printf("Strings are Equaln");
    } else {
        printf("Strings are Unequaln");
    }

    return 0;
}

Output:

Strings are Equal

Explanation: In this code, s1 and s2 are initialized with the same string “Hello”. strcmp(s1, s2) returns 0 because the strings are identical. The if condition evaluates to true, and “Strings are Equal” is printed.

Example 2: Finding Lexicographically Greater String

Here, we compare two strings where the first string is lexicographically greater than the second.

#include <stdio.h>
#include <string.h>

int main() {
    char str1[] = "zebra";
    char str2[] = "apple";

    int res = strcmp(str1, str2);

    if (res == 0) {
        printf("Strings are Equaln");
    } else if (res > 0) {
        printf("str1 is lexicographically greater than str2n");
    } else {
        printf("str1 is lexicographically smaller than str2n");
    }

    return 0;
}

Output:

str1 is lexicographically greater than str2

Explanation: “zebra” comes after “apple” in lexicographical order because ‘z’ has a higher ASCII value than ‘a’. Therefore, strcmp(str1, str2) returns a positive value, and the program correctly identifies str1 as lexicographically greater.

Example 3: Sorting an Array of Strings Using strcmp()

strcmp() is particularly useful when sorting arrays of strings. It can be used as a comparison function with sorting algorithms like qsort() from <stdlib.h>.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int compareStrings(const void *a, const void *b) {
    return strcmp(*(const char **)a, *(const char **)b);
}

int main() {
    const char *stringArray[] = {"Banana", "Apple", "Cherry", "Date"};
    int n = sizeof(stringArray) / sizeof(stringArray[0]);

    qsort(stringArray, n, sizeof(stringArray[0]), compareStrings);

    printf("Sorted array of strings:n");
    for (int i = 0; i < n; i++) {
        printf("%sn", stringArray[i]);
    }

    return 0;
}

Output:

Sorted array of strings:
Apple
Banana
Cherry
Date

Explanation:
In this example, compareStrings function utilizes strcmp() to compare two strings. qsort() then uses this comparison function to sort the stringArray lexicographically. The output displays the strings in ascending alphabetical order.

C strcmp() – Frequently Asked Questions (FAQs)

When will the strcmp() function return zero?

The strcmp() function specifically returns zero when it determines that the two strings being compared are exactly identical. This means they have the same length and contain the same sequence of characters in the same order.

What does a positive return value from strcmp() signify?

A positive return value from strcmp() indicates that the first string (s1) is lexicographically greater than the second string (s2). This could be because the first differing character in s1 has a higher ASCII value, or because s1 is longer and s2 is a prefix of s1.

What is the meaning of a negative return value from strcmp()?

A negative return value from strcmp() means that the first string (s1) is lexicographically smaller than the second string (s2). This can occur if the first differing character in s1 has a lower ASCII value, or if s2 is longer and s1 is a prefix of s2.

Can strcmp() be used to compare data types other than strings in C?

No, the strcmp() function in C is specifically designed to compare strings. It operates on null-terminated character arrays. Attempting to use strcmp() with non-string data types will lead to undefined behavior and incorrect results. For comparing other data types, you would use different comparison operators or functions appropriate for those types.

Conclusion

The strcmp() function is a cornerstone of string manipulation in C. Understanding its lexicographical comparison mechanism and return values is crucial for effectively working with strings. From simple string equality checks to complex sorting operations, strcmp() provides a robust and efficient way to compare strings in your C programs. Mastering C Compare String operations with strcmp() will significantly enhance your ability to handle text-based data and algorithms in C.